This invention relates to an arrangement for facilitating the performance of the TTL photometric function in an SLR.
Most modern SLR cameras are designed so that the camera body is adapted to accept various interchangeable components including interchangeable lenses and interchangeable viewfinders. A wide variety of viewfinder systems have been proposed heretofore. Interchangeable viewfinders include waist-level-finders, high magnification finders, and sports finders. An example of an eye level/waist level selectable finder is disclosed and claimed in a commonly assigned application entitled VIEWFINDER SYSTEM FOR A SINGLE LENS REFLEX CAMERA, filed Sept. 11, 1975 by Tetsuji Shono and bearing Serial No. 612,276. Almost all the modern SLR's have a TTL photometric function for this has been found to simplify the exposure determining operation and other operations. Various proposals have been set forth with respect to where one or more light receiving elements should be positioned with respect to the associating TTL photometric optical system. Depending on the positions at which the light receiving elements are disposed, SLR cameras may be classified into two broad types. The first type is such that, taking an eye level finder for instance, one or more light receiving elements are arranged close to the pentaprism so that, when the pentaprism is detached from the camera body for replacing the finder with another one, the light receiving elements are detached together with the pentaprism. The second type is such that one or more light receiving elements are arranged either on the rear side of the mirror or on the surface of the condenser lens at its peripheral positions so that the light receiving elements are not detached from the camera body upon disengagement of the finder. In the SLR cameras of the second type, it is unnecessary to arrange a photometric optical system on the side of the interchangeable finder. Therefore, even when the finders are interchanged, the TTL photometric function remains unchanged and the photometric conditions are unaffected. On the other hand, the SLR cameras that have a TTL photometric system on the side of the interchangeable finder involved problems. Included among these problems is the problem of effecting the circuit connection between the camera body and the attached finder, which has to be carried out after each finder interchanging operation; differences in optical systems between different finders and arrangement of light receiving elements. These difficulties are liable to have an adverse effect upon the photometric function and accuracy thereof. Moreover, when a finder having no photometric function is attached, the practical value of the camera will be much reduced because the camera will no longer be able to exhibit a photometric function, thereby making it difficult to attain an accurate film exposure.
Therefore, in circumstances in which TTL photometry is to be effected by arranging a photometric optical system and one or more light receiving elements in the interchangeable viewfinder for an SLR, it becomes a problem how to minimize the error in the interlocking diaphragm operation, deviation of the light receiving efficiency, non-uniformity in the distribution of the received light, influence of the counter-incident light coming through the eyepiece, and influence of the provision of a photometric optical system and light receiving elements over the easy finder observation. It is important to minimize the error in the interlocking diaphragm operation, not only for the interchangeable lenses for use in a camera in a TTL photometric type but also for the interchangeable viewfinders. That is, photometric error which arises in different interchangeable viewfinder results in an error in the interlocking diaphragm operation, which in turn results in an error in film exposure.
Consider now circumstances in which an exposure value EV3 (the third exposure multiple grade; magnification, X8 or 1/8) is obtained with an eye level finder wherein the difference between the full-open photometric outputs obtained by, for example, a lens of F=1.4 and 50mm focal length and another lens of F=4 and 200mm focal length forms the standard level. If the exposure value is changed to EV2.5 by replacing the finder with another one, then an error of EV0.5 appears which acts as a cause of error in the interlocking diaphragm operation. This error is most influenced by the variation in the relative position and angular position of the light-receiving elements with respect to the imaging plane. It is therefore necessary that, in each interchangeable finder, the light-receiving elements should be arranged at conjugate positions and in an equal angular position with respect to the imaging plane.
The light-receiving efficiency is a photometric conversion efficiency of a light-receiving element and shows how much output the light-receiving element develops upon receiving a light having a preselected brightness. Electrical compensation for the difference between the light-receiving efficiencies of various interchangeable finders involves a complicated photometric circuit. For this reason, interchangeable finders are preferably made in such a manner that they have an identical light-receiving efficiency. The distribution of the received light is determined by the angular position of the light-receiving elements with respect to the imaging plane. Therefore, to eliminate the danger of photometric error the light-receiving elements of the interchangeable finders have to be arranged so that they have an identical angular position to the corresponding imaging plane. The counter-incident light coming through the eyepiece will result in an error in the photometry and therefore should be eliminated. However, to perfectly shut off the counter-incident light, it is necessary to provide, for example, an eyepiece shutter and to close it at each photometric operation. This is very troublesome and inconvenient. In order to minimize the influence of the counter-incident light coming through the eyepiece without using any additional complicated means, the light-receiving elements have to be disposed at precise positions. It is also necessary to eliminate the danger that the easy finder observation is prevented by the provision of the photometric optical system and light-receiving elements.